Means and method for controlling an alkylation unit to achieve a desired feed isobutane flow rate

ABSTRACT

A control system and method controls an alkylation unit in a manner to achieve a desired feed isobutane flow rate. Trial isobutane flow rates and their effects are calculated. The trial isobutane flow rate, along with its corresponding discharge acid flow rate, that obtains a desired effect is then imposed on the feed isobutane and discharge acid, respectively.

United States Patent [1 1 Sweeney, Jr.

1 MEANS AND METHOD FOR CONTROLLING AN ALKYLATION UNIT TO ACHIEVE A DESIRED FEED ISOBUTANE FLOW RATE [75] Inventor: Donald E. Sweeney, Jr., New

Orleans, La.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Mar. 19, 1973 [21] Appl. No.: 342,650

[52] US. Cl.... 235/151.l2, 208/D1G. 1, 260/683.58 [51 Int. Cl G06g 7/58 [58] Field ofSearch ..235/151.l2, 151.1, 150.1; 444/1; 208/134, 133, DIG. 1, 141;

CONTACTOR 7 SETTLER ANALYZER DENSITY ANALYZER oswsmr ANALYZER 63 FLOW neconoen cour. E

DISCHARGE 5 ACID PROGRAM MEANS TOGRAPH MEANS MEANS OLEFINS ISOBUTANE ACID DENSITY June 4, 1974 [56] References Cited UNITED STATES PATENTS 2,929,857 3/1960 Hutto 260/683.58 X 3,018,310 l/1962 Van Pool 260/683.43 X 3,160,673 12/1964 Black ct al 260/683.58 X 3,686,354 8/1972 Hervert 260/683.43

Primary Examiner-Joseph F. Ruggiero Attorney, Agent, or FirmT. H. Whaley; C. G. Reiss [57] ABSTRACT A control system and method controls an alkylation unit in a manner to achieve a desired feed isobutane flow rate. Trial isobutane flow rates and their effects are calculated. The trial isobutane flow rate, along with its corresponding discharge acid flow rate, that obtains a desired effect is then imposed on the feed isobutane and discharge acid, respectively.

23 Claims, 13 Drawing Figures so as E| ALKYLATE E CHRQM HYDROCARBON |1 TOGRAPH MEANS CONTROL MEANS ELECTRONIC SWITCH ELECTRONIC SWITCH SHEET 7 [IF 8 mwm www

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention related to control systems and methods in general and, more particularly, to a control system and method for an alkylation unit.

2. Description of the Prior Art Heretofore control systems such as described in US. Pat. Nos. 3,733,473 (issued May 15, 1973) and 3,729,624 (issued Apr. 24, 1973) controlled the acid strength in an alkylation unit. Another US. Pat. No. 3,728,527 (issued Apr. 17, 1973) discloses a system and method for controlling an alkylation unit to achieve an optimum acid strength while an application Ser. No. 281,063, filed Aug. 16, 1972, discloses a system and method for controlling the hydrocarbon content of recycled acid in an alkylation unit. The invention of still another application Ser. No. 257,408 May 26, 1972 controls the contact temperature during the alkylation process. All of the foregoing applications are assigned to Texaco, Inc. assignee of the present invention.

The system and method of the present invention distinguishes over the aforementioned systems and methods in controlling the feed isobutane to the alkylation unit to achieve a desired operating condition for the alkylation unit.

SUMMARY OF THE INVENTION A system and method controls an alkylation unit to achieve a desired operating condition. The flow rate of the isobutane entering the alkylation unit is controlled along with one acid flow rate of the flow rates of fresh acid and discharge acid relative to the other acid flow rate to achieve the desired operating condition in accordance with control signals. A circuit provides signals corresponding to sensed operating parameters. Another circuit provides a signal corresponding to trial isobutane flow rates. A signal corresponding to a different one acid flow rate associated with each trial isobutane flow rate is also provided. A network determines an anticipated change in profit associated with the implementation of each trial isobutane flow rate and its corresponding one flow rate. A selection circuit selects the trial isobutane flow rate signal and the associated one acid flow rate signal that provides for the greatest increase in profits and the selected signals are provided as the control signals. 7

The object and advantages of the invention will appear hereinafter from consideration of the detailed description which follows, taken together with the accompanying drawings wherein two embodiments of the aforementioned invention are illustrated by way of the example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified block diagram of a control system, constructed in accordance with the present invention, for controlling an alkylation unit, which is also shown in partial schematic form, providing a hydrocarbon product for further processing.

FIGS. 2 through 113 are detailed block diagrams of the programming means, the alkylate signal means, the A computer, 11 computer, a portion of the olefin signal means, the 8,, computer, the G computer, the F computer, the F computer and the control means shown in FIG. 1.

FIG. 12 is a detailed block diagram of the R computer shown in FIG. 11.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a portion ofan alkylation unit in which olefins are reacted with an isoparaffin, such as isobutane; in the presence of a catalyst, such as sulphuric or hydrofloric acid, and which is hereinafter referred to as acid, to form a higher molecular weight isoparafi'm. For purposes of explanation, the acid in the following description shall be sulphuric acid. The olefin may be butylenes, propylene, or a mixture of butylenes and propylene.

The olefins and isobutane are provided in lines 3 and 3A, respectively, and enter a contactor 4, by way of a line 6, where the olefins and isobutane are contacted with recycle acid entering by way of a line 7. Contactor 4 provides an acid-hydrocarbon mix by way of a line 8 to an acid settler 12. Settler 12 separates a hydrocarbon product, which includes alkylate, from the acid and the hydrocarbon product is provided through a line 14 while the acid is removed by way of a line 16. It should be noted that the acid from settler 12 is an acid which is enriched with some hydrocarbon that has not been thoroughly separated by the action of settler l2. Acid settler 12 may be the only acid settler in the unit or it may be the last acid settler in a group of acid settlers. Inside acid settler 12, a hydrocarbon phase is separated from an acid phase by an interface level.

Fresh acid enters line 16 by way of a line 17 as needed to maintain a desired acid strength. A pump 20 pumps the acid from line 16 into line 7. A portion of the acid in line 7 is discharged by way of a line 21 leaving the recycle acid to be provided to contactor 4. The discharged acid may be provided to another alkylation unit or disposed of.

It has been found that the effect of percent isobutane in the hydrocarbon product on the octane quality, the acid consumption and profits can be estimated by a series of equations.

The existing acid consumption A for the alkylation unit can be determined from Equation 1.

n AI1/ CB CK) where R and R are the existing flow rates of the discharge acid and the hydrocarbon product, respectively, V is the existing volume fraction of the alkylate product in the hydrocarbon product stream, and a is a constant, and may have a value of 0.3213, which is to convert the discharge acid rate from barrels per hour to tons per hour.

The existing olefin space velocity 8,, is determined from Equation 2 (2) 5 where R is the actual flow rate of the hydrocarbon where R and R are the flow rates of the olefin charge stream and recycle acid, respectively, V is the volume fraction of the olefins in the olefin charge stream, Cy is the volume of contactor 4 in barrels and is a predetermined value, and H is the existing hydrocarbon content in the recycle acid.

The hydrocarbon content H of the recycle acid is determined from Equation 3 where D D and D are the densities of the fresh acid, of the recycle acid and of the hydrocarbon product, respectively.

The existing octane quality value 0,, is determined from Equation 4 GB: b1 S +b2 1n(100I lb where b,, 12 and [2, are constants associated with a particular alkylation unit, by way of example maybe 6.5, 8.65 and 30.0 respectively, and V is volume fraction of isobutane in the hydrocarbon product.

A factor F for correcting acid consumption for changes in percent isobutane in the hydrocarbon product may be determined from Equation 5 irm I HU where c is a constant associated with a particular alkylation unit, which by way of example may have a value of 1.052.

A factor s'n for correcting acid consumption for changes in the olefin space velocity is determined using Equation 6 where d, through d, are constants associated with a particular alkylation unit and which, by way of example, may have values of 0.942361, O.l8270208, 0023882011 and 0.0012078644, respectively, and P is the ratio of volume percent propylene to the volume percent total olefins in the olefin charge stream.

The change AM in the feed isobutane is determined from Equation 7 7. RgmR where R is the existing flow rate of the isobutane and R is a calculated trial flow rate of the isobutane for a new condition.

A calculated flow rate R for the hydrocarbon product can be determined from Equation 8.

product.

Similarly, a calculated volume fraction of isobutane in the hydrocarbon product can be determined from Equation 9.

cc (AMVM RCB cn)/ ('c where V is the volume fraction of isobutane in the feed isobutane in line 3A and V is the volume fraction of isobutane in the hydrocarbon product in line 14.

The change in octane quality AQ may be determined from Equation 10.

new condition. The calculated space velocity S may be determined from Equation 12.

S IOOR VAR R )/C R -H) The change in acid consumption AA may be calculated from the Equations 13, 14 and 15 where F M and F are factors for correcting acid consumption for the new condition to account for changes in percent isobutane in the hydrocarbon product and for changes in the olefin space velocity, respectively.

The change in earnings E may be determined from the following Equation 16.

EC DQAADA AMDM/RCBVCK where 0 D and D are economic values associated with the octane quality, the acid consumption and the isobutane, respectively. The new calculated discharge acid flow rate R, may be determined from Equation 17.

In selecting a desired isobutane content, a control system, as hereinafter described, determines the effect of different trial isobutane flow rates. Each trial isobutane flow rate R is determined in accordance with Equation 18.

where J is any positive integer, j is a selected integer from zero to J and AR is the maximum allowable change in the isobutane flow rate inline 3A.

Referring to FIG. 1 the control system there shown senses existing parameters during operating of the alkylation unit and calculates new values for a predetermined change in some of the parameters of the alkylation unit. The control system continues to calculate new values until certain predetermined limits are reached at which time the control system will determined the flow rates of the discharge acid and the isobutane and control the alkylation unit accordingly. A flow rate sensor 24 senses the flow rate of the hydrocarbon product in line 14 to provide-a signal 15,, corresponding to the sensed flow rate, to an A 8 computer 28.

Chromatograph means 3 0 samples the hydrocarbon product in line 14 and provides a signal E to alkylate signal means 38 and a pulse signal E to programming means 33.

Referring also to FIG. 2, each pulse in signal E corresponds to a peak in signal E Signal E is applied to an AND gate in a control pulse circuit 41 which is controlled by an output from a logic decoder 42. When the output from decoder-42 is a high level direct current output, AND gate 40 passes pulses E to a counter 44. Outputs from counter 44, corresponding to the count, are applied to logic decoder 42 which provides a plurality of outputs to one shot multivibrators 45. Different counts in counter 44 are decoded by decoder 42 to trigger different one shot multivibrators 45 causing one shots 45 to provide a plurality of control pulses E through E When the count in counter 44 reaches a predetermined count, the output from logic decoder 42 to AND gate 40 goes to a low level, disabling AND gate 40 to prevent counter 44 from counting. The sampling operation is effectively recycled by the occurence of a reset pulse E applied to counter 44 and to chromatograph means 30 as hereinafter described.

The E, pulse from one shot 45 is also applied to a one shot multivibrator which is used as a time delay to provide for the calculation time necessary to the determination of a signal E corresponding to the percent volume of alkylate in the hydrocarbon product. One shot 50 provides a pulse output to trigger another multivibrator 51 which in turn provides a control pulse E Referring now to FIGS. 1 and 3, the peaks of signal E from chromatograph means 30 correspond to the different constituents of the hydrocarbon product in line 14. Sample and hold circuits 55 through 551 in alkylate signal. means 38 are controlled by control pulses E through E respectively, from programming means 33 to hold different peaks of signal 8,. The following table relates a particular sample and hold circuit to a corresponding constituent.

Circuit Constituent Circuit CONSTITUENT 55 Ethane 55E Normal Pentane 55A Propane 55F Propylene 55B lsohutane 55G Buytlenes 55C Normal butane 55H Pentylenes 55D lsopentane 551 All compound with six or more carbon atoms The outputs from sample and hold circuits 55 through 551 are applied to multipliers 56 through 561, respectively, where they are multiplied with direct current voltages V through V,,, respectively, corresponding to the various chromatograph means 30 scaling factors pertaining to the particular constituents. By way of example, the voltages V through V may correspond to 0.02, 0.2 1.0, 0.2, 0.15, 0.02, 0.2, 0.10, 0.02 and 0.10 volts, respectively. The signals from multipliers 56 through 561 are sampled and held by circuits '57 through 571, respectively, in response to pulse signal E,,- from programming means 33. Sample and hold circuits 57 through 571 are used so that outputs corresponding to various constituents of the charge olefin may be presented simultaneously to summing means 58. Summing means 58 provides a normalizing signal. The output from multiplier 57], corresponding to the constituent of the hydrocarbon product having all compounds of six or more atoms, is applied to a divider 60 where it is divided by the normalizing signal from summing means 58 to provide a signal E corresponding to the volume fraction V of alkylate in the hydrocarbon product. Signal E is applied to A,, computer 28.

A divider 61 divides the signalfrom sample and hold circuit 578 with the normalizing signal from summing means 58 to provide a signal E corresponding to the volume fraction V of isobutane in the hydrocarbon product.

Referring to F168. 1 and 4, a flow rate sensor 63 in line 21 provides a signal E corresponding to the flow rate R of the discharge acid to A computer 28. A multiplier 64 multiplies signal E, with a direct current voltage V corresponding to the constant a in Equation 1, to provide a signal to a divider 65 corresponding to the term aR in Equation 1. Another multiplier 66 multiplies signals E,, E to provide a signal to divider 65 corresponding to the term R V Divider 65 divides the signal from multiplier 64 with the signal from multiplier 66 to provide a signal E corresponding to the existing acid consumption A Referring to FIGS. 1 and 5, density analyzers 70, 70A and 70B sense the densities of the hydrocarbon product in line '14, the recycle acid in line 7 and the fresh acid in line 17, respectively, and provide signals E E and E respectively, to an H computer 72. H computer 72 provides a signal E corresponding to the hydrocarbon content H of the recycle acid in accordance with signals E, E and E and equation 3. Subtracting means 74 and 76 subtract signals E and E respectively, from E to provide signals, corresponding to the terms (D -D and (D,-D to a divider 75. It provides a signal to a multiplier 77 where it is multiplied with a direct current voltage V corresponding to the term in Equation 3 to provide signal E,,, I

Referring to FIG; 1, chromatograph means 82 samples the olefin stream in line 3 to provide a constituent signal E to olefin signal means 83 and a pulse signal E to programming means 33. Chromatograph means 82 is similar to chromatograph means 30.

Olefin signal means 83 is similar to alkylate signal means 38 but is controlled by pulses E through E from a control pulse circuit 41A in programming means 33. Control pulse circuit 41A is identical to circuit 41. The difference between the two signal means may be seen in FIG. 6. It should be noted that only the portion of olefin signal means 83 that is different from alkylate signal means 38 is shown in FIG. 6 to avoid unnecessary repetition of operation already explained. The outputs from sample and hold circuits 57F through 571 are summed by summing means 86 to provide a signal to dividers 87, 88. Divider 87 divides the signal from summing means 86 with the signal from summing means 58 to provide a signal E corresponding to volume fraction V of olefins in the olefin stream in line 3. Divider 88 divides the output from sample and hold circuit 57F with the signal from summing means 86 to provide a signal E corresponding to the ratio of propylene to olefins in line 3.

Chromatograph means 82A is similar to chromatograph means 82 in operation. Means 82A samples the isobutane stream in line 3A and provides signals E and E to isobutane signal means 83A and to programming means 33, respectively, and is periodically reset by the reset pulse from programming means 33.

lsobutane signal means 83A is similar to olefin signal means 83. Signal means 83A does not provide signals E and E therefore there are no elements present in signal means 83A corresponding to summing means 86 and dividers 87 and 88. In providing signal E which corresponds to the isobutane content, signal means 83A does include a divider dividing a signal from a sample and hold circuit, which corresponds to circuit 578 in signal means 83, with a signal from summing means corresponding to summing means 58.

Pulses E through E are provided by a control pulse circuit 41,; in programming means 33. Control pulse circuit 41B is identical to control pulse circuit 41A.

Referring to FIGS. 1 and 7, a sensor 90 in line 7 provides a signal E corresponding to the sensed flow rate R of the recycle acid to an 5,; computer 91. Another sensor 93 senses the olefin flow rate in line 3 and provides a signal E to S computer 91. Computer 91 also receives signals E E. and E and provides a signal E corresponding to the existing space velocity of the olefin in accordance with signals E,, E,,, E E, and E and Equation 2. A multiplier 97 in 5,, computer 91 multiplies signal E, with voltage V to provide a signal, corresponding to the term 100 R in Equation 2, to a multiplier 98. Multiplier 98 multiplies the signal from multiplier 97 with signal E to provide a signal, corresponding to the term 100 R V to a multiplier 99. Summing means 100 sums signals 15., E to provide a signal to multiplier 99 corresponding to the term R R Multiplier 99 in turn multiplies the two signals to provide a signal, corresponding to the numerator in Equation 2, to a divider 104.

A multiplier 105 multiplies signal E with a direct voltage current V.,, corresponding to the predetermined volume C lof contactor 4, to provide a signal to another multiplier 108. Subtracting means 109 subtracts signal E, from direct current voltage V to provide a signal to multiplier 108. Multiplier 108 multiplies the signals from multiplier 105 and subtracting means 109 to provide a signal corresponding to the term C R (100-H) to divider 104. Divider 104 divides the signal from multiplier 99 with the signal from multiplier 108 to provide signal E Referring now to FIGS. 1 and 8, G computer provides a signal E corresponding to the existing octane quality value, G Computer 115 includes a multiplier 116 receiving signal E which effectively squares signal E to provide a signal corresponding to the term S,, to another multiplier 117. Multiplier 117 multiplies the output from multiplier 116 with a direct current voltage V corresponding to the constant I), in Equation 4. A multiplier 120 multiplies voltage V with signal E to provide a signal corresponding to the term 100 V to summing means 121. Summing means 121 sums the signal from multiplier 120 with a direct current voltage V corresponding to the constant b to provide a signal to a logrithmic amplifier 124.

It will be noted in equation 4 that there is a term In (100 V +b Logrithmic amplifier 124 cooperates with another logrithmic amplifier 125 and a divider 126 to provide a signal corresponding to that expression. The natural log of a term is obtained by dividing the log to the base of 10 of that term with the log to the base 10 of e. Thus, logrithmic amplifier 124 provides a signal corresponding to the log to the base 10 of the output from summing means 121 while logrithmic amplifier 125, receiving a direct current voltage V corresponding to the mathematical constant e, provides a signal corresponding to the log to the base 10 of the constant e. Divider 126 divides the output from amplifier 124 with the output from logrithmic amplifier125 to provide a signal to a multiplier 128 corresponding to the term In 100 V -,,+b;,). Multiplier 128 multiplies the output from divider 126 with a direct current voltage V, corresponding to the constant 12 Summing means 130 sums the output from multipliers 117 and 128 to provide signal E Referring now to FIGS. 1 and 9, the olefins space velocity signal E is also applied to an F computer which also receives signal E from olefin signal means 83 and provides a signal E corresponding to the term F in Equation 6. Signal E is applied to a divider 136 and to subtracting means 137 in computer 135.. Sub tracting means 137 subtracts signal E from a direct current voltage V corresponding to the term 1 in Equation 6, to provide an output, corresponding to lP,,, to divider 136. Divider 136 divides signal E by the output from subtracting means 137 to provide a signal corresponding to the term P,,/ l-P,,. The signal from divider 136 is applied to multipliers 140 through 143. Multiplier 140 effectively squares the signal from divider 136 while multiplier 141 multiplying the signal from multiplier 140 with the signal from divider 136 effectively cubes the signal from divider 136. Multiplier 142 multiplying asignal from multiplier 141 with the signal from divider 136 effectively raises the signal from divider 136 to the fourth power. Multiplier 143 multiplies the output from divider 136 with the direct current voltage V corresponding to the constant d Multipliers 144, 145 and 146 effectively multiply the outputs from multipliers 140, 141 and 142, respectively, with direct current voltages V 1, V and V respectively, corresponding to the constants d d; and d, to provide outputs to summing-means 149 which also receives the output from multiplier 143. Summing means 149 provides a sum signal to a multiplier 150.

A multiplier 151 effectively squares signal E and provides a signal, corresponding to S to multiplier 150 where it is multiplied with the output from summing means 149. A multiplier 153, a logrithmic amplifier 154, an operational amplifier 155 and a feedback element 156 effectively raise the mathematical constant e to the power of the signal from multiplier 150. Voltage V corresponding to the mathematical constant e, is applied to logrithmic amplifier 154 which in turn provides an output to multiplier 153. The output from multiplier 153 is multiplied with the output from multiplier 150 to provide a signal corresponding to the term in the wavy brackets log e of Equation 15. The output from multiplier 153 is applied to operational amplifier 155 having feedback element 156. Feedback element 156 which is a function generator and may be of the type manufactured by Electronic Associates, as their part number PC 12, operates as an antilog circuit to provide signal E An FM computer 160, shown in detail in FIG. 10, provides a signal E corresponding to the factor F in accordance with signal E and Equation 5. A multiplier 161 in computer 160 multiplies signal E with voltage V to provide a signal, corresponding to the term 100 V to a logrithmic amplifier 162. The output from logrithmic amplifier 162 is provided to a multiplier 164 where it is multiplied with a direct current voltage V corresponding to the constant c, to provide an output corresponding to log 100 V The output is applied to an anti-log circuit, comprising an operational amplifier 155A having a feedback element 156A, which provides signal E Referring to FIG. 1, the interface, separating the acid and hydrocarbon phases, in acid settler 12 is maintained at a predetermined level by a level recorder controller 174 receiving a level signal from a level sensor 175. Level recorder controller 174 operates a valve 176 in line 17 controlling the fresh acid flow rate. The discharge acid in line 21 is controlled by a valve 180 receiving a signal from flow recorder controller 181, which in turn receives signal E from flow rate sensor 63. The set point in flow recorder controller 181 is positioned, as hereinafter described, to a desired discharge acid flow rate. Controller 181 then provides a signal corresponding to the difference between the sensed discharge acid flow rate and the desired discharge acid flow rate to control valve 180 so the flow rate of the discharge acid substantially corresponds to the desired discharge acid flow rate. When the discharge acid flow rate is increased the interface level in acid settler 12 initially decreases causing controller 174 to'provide a different signal to valve 176. Valve 176 increases the fresh acid flow rate to restore the desired interface level. When the discharge acid flow rate is decreased, the interface level increases causing controller 174 to decrease the fresh acid flow rate until the interface level is restored. Y

The isobutane flow rate in line 3A is controlled by a valve 184 and a flow recorder controller 185. Flow recorder controller 185 has its set point positioned, as hereinafter described, to a desired flow. rate for the isobutane. Flow rate controller 185 receives signal E from flow rate sensor 171 and provides a signal to valve 184 corresponding to the difference between the sensed flow rate and the desired flow rate for the isobutane. Valve 184 is then controlled accordingly so that the flow rate of the isobutane in line 3A assumes the desired isobutane flow rate.

The control system controls the alkylation unit by determining different isobutane flow rates, and calculating the effect on the alkylation unit and the corresponding costs from those effects, and selecting the isobutane flow rate providing the most economical operation. Referring now to FIGS. 1, 11A, 11B and 12 control means receives signals E E E E E E 12 14 15 11, 18 20 21 and 22 and Provides S gnals E and E6 corresponding to a desired discharge acid flow rate and to a desired isobutane flow rate, respectively. An RMj computer 171, shown in F 1G. 12, provides a signal E corresponding to a trial flow rate R for the isobutane. Various trial isobutane flow rates RM,- are determined in accordance with Equation 18.

The control operation is initiated when an operator closes a switch 174 in R computer 171 thereby applying a high level directcurrent voltage V to an AND gate 175. AND gate 175 is enabled by the passed voltage V to pass timing pulses from clock means 178 to a counter 183. As counter 183 counts the timing pulses passed by AND gate 175, a logic decoder 184, decodes the count in counter 183 to provide different outputs to control electronic switches through 185C. Electronic switches 185 through 185C receive direct current voltages V V through V corresponding to different integers such as O, l, 2 and 3. Thus, for a trial isobutane flow rate logic decoder 184 decodes the count in counter 183 to'activate switch 185 passing voltage V corresponding to 0 to a multiplier where it is multiplied with voltage V corresponding to 2 to provide an output corresponding to the term 2j in Equation 18. The output from multiplier 190 is divided by the voltage corresponding to the term J, which in this instance is voltage V by a divider 191. The output from divider 191 has voltage V subtracted from it by subtracting means 192 which provides a signal corresponding to the expression 2j/J l in Equation 18. A direct current voltage V corresponding to the maximum allowable change in the isobutane flow rate AR is multiplied with the signal from subtracting means 192 by a multiplier 193. The output is provided to summing means 194 where it is summed with signal E to provide signal E As the count in counter 183 increases, logic decoder 184 renders switches 185, 1858 and 185C nonconductive to block voltage V V and V respectively, and switch 185A conductive to pass voltage V corresponding to j having the value of 1, so that signal E corresponds to a new trial isobutane flow rate R Similarly, switch 1858 is rendered conductive while switches 185, 185A and 185C are rendered nonconductive so-that voltage V is used to calculate a new trial isobutane flow rate and again it is repeated for the rendering of switch 185C conductive while switches 185 to 1858 are rendered non-conductive so that voltage V is used to calculate a new trial isobutane flow rate.

It is necessary that an earnings signal be set to a zero value momentarily for reasons hereinafter disclosed, for each subsequent trial isobutane flow rate. This is accomplished by a pulse E provided by R computer 171 as will be explained hereinafter. A plurality of one shot multivibrators 200 through 200C are triggered by negative going outputs from logic decoder 184. The pulses provided by one shots 200 through 200B pass through OR gates 195, 196 trigger a one shot multivibrator 201.

The resulting pulse from multivibrator 201 is inverted by an inverter 202 to become a negative going pulse E The pulse from multivibrator 200C also generators an E pulse by passing through OR gate 196 to trigger multivibrator 201.

The pulse from multivibrator 200C triggers another one-shot multivibrator 205 causing it to provide reset pulse E Reset pulse E resets counter 183, chromatograph means 30,82 and 82A, control pulse circuits 41,41A and 418, a flip-flop 197 and other storage registers as hereinafter described.

Flip-flop 197 permits the alkylation unit to be controlled externally during the initial cycle of operation as hereinafter explained. The passed voltage V from switch 174 triggers flip-flop 197 to a set" state. Flipflop 197 provides signal E at a high level while in a set state and at a low level while in a clear state. Flip-flop 197 changes back to the clear" state in response to reset pulse E Flip-flop 197 remains in the set" state until switch 174 is opened and closed again.

Referring again to FIGS. 1, 11A and 11B, for a current trial isobutane flow rate, subtracting means 220 subtracts signal E from signal E to provide a signal E corresponding to AM in accordance with Equation 7. Summing means 222 sums signal E with the AM signal E to provide a signal E corresponding to the calculated hydrocarbon product flow rate R A multiplier 224 multiplies signal E E to provide a signal corresponding to the term AMV A multiplier 225 multiplies signal E E together to provide a signal corresponding to the term R V Summing means 226 sums the signal from multipliers 224, 225 to provide a signal, corresponding to the numerator of Equation 9, to a divider 230 where it is divided by signal E Divider 230 provides a signal E corresponding to the volume fraction of isobutane in the hydrocarbon product.

Signal E is applied to an F computer 232 and a G computer 233. The F Computer 232 is identical to F computer 160 shown in FIG. 10 except that signal E is used in lieu of signal E Computer 232 provides a signal E corresponding to the calculated factor, F for correcting acid consumption for changes in percent isobutane in the hydrocarbon product.

An S computer 235, receiving signals E E E E and E provides a signal E corresponding to a calculated olefin space velocity, Sp, for the current trial isobutane flow rate. Computer 235 is identical to S computer 91 except signal E is used in lieu of signal E Signal E is applied to G computer 233.

G computer 233 is identical to G computer 115 except that signals E and E are used in lieu of signals E and E respectively. G(' computer 233 provides a signal E40. corresponding to a calculated octane quality,-G for-the current trial isobutane flow rate. Subtracting means 240 subtracts signal E from signal E to provide a signal E corresponding to the change AQ in octane quality in accordance with Equation 10.

A change AA in acid consumption is determined as follows. An F computer 241, which is identical to F computer 135 except that computer 241 uses signal E in lieu of signal E provides a signal E corresponding to he factor F for correcting acid consumption for changes in olefin space velocity for the current trial isobutane flow rate.

Multipliers 243, 244 multiply signals E E with signals E and E respectively, to provide signals corresponding to the terms F F and F -F respectively, in Equation 13. A divider 245 divides the signal from multiplier 244 by the signal from multiplier 243 to provide an output to subtracting means 250. Subtracting means 250 subtracts voltage V from the signal provided by divider 245 to provide a signal corresponding to the bracketed portion of equation 13 A multiplier 251 multiplies signal E with the signal from subtracting means 250 to provide a signal E corresponding to the change AA in acid consumption resulting from the trial isobutane flow rate.

In regard to the computation of increased profits in accordance with Equation 16, multipliers 256, 257 and 258 multiply signals E E. and E respectively, with direct current voltages V V and V respectively, corresponding to the economic values D D D respectively, associated with the octane quality, the acid consumption, and the isobutane content. Another multiplier 260 multiplies signals E, E, together to provide a signal corresponding to the term R V A divider 261 divides the signal from multiplier 256 with the output from multiplier 260. Subtracting means 264 subtracts the output provided by multiplier 258 from the output provided by multiplier 257 to provide a signal to subtracting means 265. The output from divider 261 is subtracted from the signal provided by subtracting means 264 to provide a signal E corresponding to the expected increased profit E which would result from changing the isobutane stream flow rate from its existing rate to the trial flow rate.

To maintain the chemical reltionship of the alkylation process, the discharge acid flow rate has to be changed in accordance with a change in the isobutane flow rate so that for each trial isobutane flow rate there is a corresponding calculated discharge acidflow rate R Summing means 270 sums signals E E to provide a signal corresponding to the bracketed portion of squati 71 Th si a 59m .sumrni emsa iz i multiplied with the signal from multiplier 260 by another multiplier 272. The product signal from multiplier 272 is supplied to a divider 273 where it is divided by direct current voltage V to provide a signal E corresponding to a calculated flow rate for the discharge acid associated with the current trial isobutane flow rate.

Since it cannot be determined beforehand which trial isobutane flow rate would yield the greatest profit increase, it is necessary that the profit increase for the current trial isobutane flow rate be compared with a previous maximum increase profit and the greater profit increase retained. This procedure is repeated for all trial isobutane flow rates so that the trial isobutane flow rate, and its corresponding discharge acid flow yielding the greatest profit increase for the alkylation unit may be used. This is accomplished by the cooperation of an electronic switch 280, an analog-to-digital converter 281, a comparator 284, a plurality of transfer AND gates 285, a storage register 286, a digital-toanalog converter 290, and one shot multivibrators 292 and 293. Signal E corresponding to the profit increase for current trial isobutane flow rate is applied to switch 280 and converter 281. Electronic switch 280 has one input connected to ground and is controlled by signal E from R computer 171. Switch 280 is in effect a single pole double throw switch so that when a negative going E pulse occurs, switch 280 effectively applies a zero potential to comparator 284. During the absence of a pulse E switch 280 passes signal E to comparator 284.

The digital outputs from converter 281 are applied to transfer AND gates 285. AND gates 285 transfer the outputs from converter 281 to storage register 286 upon the occurence of a transfer pulse as hereinafter explained. The outputs of storage register 286 are applied to digital-to-analog converter 290 where they are converted to an analog signal E which is applied to comparator 284. Storage register 286 is used to store the maximum profit increase signal.

In effect, comparator 284 compares the profit increase for the current trial isobutane flow rate with the previous maximum profit increase. When the current profit increase is less than the previous maximum profit increase, the control system goes on to the next trial isobutane flow rate. When the current profit increase is greater than the previous maximum profit increase stored in storage register 286, the current profit increase replaces the previous maximum profit increase so that at all times storage register 286 contains the maximum profit increase for all previous trial isobutane flow rates.

Comparator 284 compares signal E with the passed signal from switch 280 during the absence of pulse E Comparator 284 provides a high level output when signal E is more positive or equal to the signal provided by switch 280 and a low level output when signal E is more negative than the signal provided by electronic switch 280. For the conditionthat the current profit increase is not greater than the previous maximum profit increase, comparator 284 provides a high level output which has no effect. The occurence of an E pulse causes switch 280 to provide a substantial zero signal which again causes comparator 284 to provide a high level output.

For the condition that the current profit increase is greater than the previous maximum profit increase, signal E passed by switch 280 is more positive than signal E causing comparator 284 output to go to a low level thereby triggering one-shot multivibrator 292. One shot 292 provides a clear pulse which clears storage register 286. The pulse from one shot 292 triggers one shot multivibrator 293 causing it to provide a transfer pulse to AND gates 285, transferring the current profit increase to register 286 as the previous maximum profit increase.

Should the next successive profit increase signal also be greater than the signal E the occurence of pulse E causes switch 280 to provide a substantially zero signal to comparator 284 causing comparator 284 to provide a high level output. Since it is at a high level output, when the next successive profit increase signal occurs, comparator 284 changes to a low level output. If it were not for the applying a zero potential to comparator 284, the output from comparator 284 would not change to a low level since it would already be at a low level from the previous profit increase signal and would not cause the next successive profit increase signal to be entered into register 286.

Whenever a profit increase signal is entered into storage register 286, it is necessary that the trial isobutane flow rate and its corresponding discharge acid flow rate associated with that particular profit increase signal also be stored so that upon completion of operation,

the stored flow rate signals may then be used to control the alkylation unit. In this regard, the current trial isobutane flow rate signal E is applied to an analog-todigital converter 300 where it is converted to digital signals and applied to transfer AND gates 301. Transfer AND gates 301 are controlled by the occurence of a transfer pulse from one shot multivibrator 293 to pass the digital signals from converter 300 to a storage re gister 302. The digital signals are stored in register 302 which has been cleared by the clear pulse from one shot 292. Similarly, signal E corresponding to the discharge acid flow rate is effectively stored in storage register 302A through the cooperation of converter 300A and transfer AND gates 301A. Upon the completion of the operation, it is necessary that the flow rate signals stored in the registers 302, 302A be stored in other registers so that the calculations for optimum isobutane and discharge acid flow rates may be repeated without affecting the operation of the alkylation unit. Reset pulse E occurring at the end of one sequence of calculations as hereinbefore explained, triggers a one shot multivibrator 305. One shot 305 acts as a time delay while reset pulse E is being used to clear storage registers 306 and 306A. One shot multivibrator 305 provides a transfer pulse to transfer AND gates 307 and 307A to effectively transfer the contents of registers 302 and 302A, respectively, to storage registers 306 and 306A, respectively. Storage register 306 outputs are converted to an analog signal E corresponding to the desired isobutane flow rate, by a conventional type digital-to-analog converter 310. Similarly, a digital-toanalog converter 310A convertsthe outputs from storage register 306A to an analog signal E corresponding to a desired discharge acid flow rate.

Referring again to FIGS. 1, 11A and 11B, signals E E are applied to electronic switches 315 and 315A, respectively, receiving direct current voltages V and V respectively. Direct current voltages V V correspond to initial flow rates for the isobutane in line 3A and the discharge acid in line 21. When signal E from flip-flop 197 in R computer 171 is a high level, electronic switches 315 and 315A pass direct current voltages V and V;,,, respectively, to flow recorder controllers and 181, respectively. Voltages V V position the set points of controllers 185 and 18], respectively, so that there is an initial flow rate for the isobutane and for the discharge acid. The occurrence of a first reset pulse E causes flip-flop 197 to cause signal E to go to a low level. A low level signal E causes switches 315, 315A to pass signals E and E to the flow recorder controller 185 and 181, respectively, so that the discharge acid flow rate assumes the desired discharge acid and isobutane flow rates.

Although the present invention as heretofore described is shown as controlling the isobutane and discharged acid flow rates it would be obvious to one skilled in the art that the discharged acid flow rate may be controlled as a function of the interface level in acid settler l2 and the fresh acid flow rate would then be controlled in accordance with the determination of the isobutane flow rate.

Although the system of the present invention has been shown using analog computer it would be obvious to one skilled in the art to use a general purpose digital computer programmed in accordance with the equations and requirements, heretofore stated, to control the isobutane flow rate and the discharged acid or fresh acid flow rate. In such a system signals E E E E E73, E10, E11, E12, E14, E17, E7, E and E22 are Converted to digital signals by conventional type analog converters while the outputs from the general purpose digital computer are converted to analog signals by conventional type digital-to-analog converters to provide signals E and E The system of the present invention as heretofore described controlled the discharge acid flow rate and the isobutane flow rate in an alkylation unit to determine the optimum flow rates for the acid and the isobutane. Trial isobutane flow rates are determined and various parameters determined in accordance with each trial isobutane flow rate. The isobutane flow rate resulting in the maximum increased profit is then used to control the alkylation unti along with an associated discharge or fresh acid flow rate.

What is claimed is:

l. A system for controlling an alkylation unit to achieve a desired operating condition, said alkylation unit includes a contactor wherein olefin and isobutane streams, entering the contactor. are contacted in the presence of acid and said contactor provides an acidhydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and acid, a portion of the separated acid is discharged while the remainder of the separated acid along with fresh acid entering the alkylation unit is recycled to the contactor, comprising means for controlling the isobutane flow rate and one acid flow rate of the fresh acid and discharge acid flow rates relative to the other acid flow rate in accordance with control signals; means for sensing certain operating parameters of the alkylation unit; means for providing a signal corresponding to trial isobutane flow rates R means for providing a signal corresponding to trial one caid flow rates R associated with the isobutane flow rates; means for determining an anticipated change in profits associated with the implementation of each tril isobutane flow rate and its corresponding one acid flow rate and providing a corresponding signal, said profit change determining means includes means for providing signals corresponding to economic values D D and D. associated with the octane rating of the alkylate, and acid and the isobutane, respectively. means for providing a signal corresponding to a change AQ in octane rating. means for providing a signal corresponding to a change AA in acid consumption, means for providing a signal corresponding to a change AM in the isobutane, and means for providing the profit change E signal in accordance with the signals from the A0, AA, AM, D and D signals and signals from the sensing means corresponding to the flow rate R of the hydrocarbon product and the following equation:

and means connected to the control means and to the profit determining means for selecting the trial isobutane flow rate and its corresponding one acid flow rate that provides for the greatest increase in profits in accordance with the profit signal and providing the selected signals as the control signals to the control means.

2. A system as described in claim 1 in which the AQ signal means includes G signal means for providing sig nals corresponding to quantities Op and G and means for providing the AQ signal in accordance with the G G signals and the following equation:

3. A system as described in claim 2 in which the G signal means includes means for providing a signal corresponding to a quantity S means for providing a signal corresponding to a quantity S means for providing a signal corresponding to a calculated fraction volume V of isobutane in the hydrocarbon product, means connected to the 8,; signal means and to the sensing means for providing the 6,, signal in accordance with the 8,, signal, a signal from the sensing means corresponding to the volume fraction V of isobutane in the hydrocarbon product and the following equation:

where b b and b-,; are constants, and means connected to the S(' signal means and to the V signal means for providing the G signal in accordance with the S and V signals and the following equation:

4. A system as described in claim 1 in which the AA signal means includes means for providing a signal corresponding to the existing acid consumption A means for providing a signal corresponding to a quantity F means for providing a signal corresponding to a quantit) 17.57}, means for providing a signal corresponding to a quantity F means for providing a signal corresponding to a quantity F and means connected to the A F F F Mn and F signal means for providing a signal corresponding to the change AA in acid consumption in accordance with the A F F F F signals and the following equation:

5. A system as described in claim 4 in which the A signal means includes means connected to the sensing means for providing a signal corresponding to the acid consumption A in accordance with the signals from the sensing means corresponding to the discharge acid flow rate R the hydrocarbon product flow rate R and the volume fraction V of alkylate in the hydrocarbon product and the following equation:

n AH/ ea ex where a is a constant.

6. A system as described in claim 5 in which the F signal means includes means for providing a signal corresponding to the ratio P,, or propylene to olefins, and means connected to the S and P signal means for providing the F signal in accordance with the S and P signals and the following equation:

where d through d,, are constants, and the F signal means includes means connected to the S signal means and to the P signal means for providing the sig nal corresponding to the quantity F in accordance with the S(' signal and the P signal and the following equation:

7. A system as desscribed in claim 6 in which the S signal includes means connected to the sensing means for providing a signal corresponding to the hydrocarbon content H on the recycle acid in accordance with signals from the sensing means corresponding to the sensed densities D D and D corresponding to the densities of the fresh acid, of the recycle acid and of the hydrocarbon product, respectively, and the following equation:

and means connected to the H signal means and to the sensing means for providing the signal corresponding to the quantity S in accordance with the H signal and signals from the sensing means corresponding to the flow rates R R and R of the oelfins, the recycle acid and the hydrocarbon product, respectively, and to the volume fraction V, of olefins in the olefin stream and the following equation:

where C y is the volume of the contactor, means for providing a signal corresponding to a calculated flow rate R for the discharge acid; and the S signal means includes means connected to the H signal means, to the sensing means and to the R signal means for providing in accordance with the R R R and V signals and the following equation:

8. A system as described in claim 7 in which the F signal means includes means connected to the sensing means for providing the F signal in accordance with a signal from the sensing means corresponding to the volume fraction V of isobutane in the hydrocarbon product and the following equation:

m; (ray where c is a constant; and the F signal means includes means for providing a signal corresponding to a calculated volume fraction V of isobutane in the hydrocarbon product, and means connected to the V signal means for providing the F in accordance with the V signal and the following equation:

9. A system as described in claim 8 in which the V signal means includes means connected to the R signal means, to the AM signal means and to the sensing means for providing the V signal in accordance with the AM, R R V signals and a signal from the sensing means corresponding to the sensed volume fraction V, of isobutane in the isobutane stream and the following equation:

AM MF MB- 11. The system as described in claim 1 in which the trial isobutane flow rate signal means is connected to the sensing means and includes means for providing a signal corresponding to a maximum flow rate change AR for the isobutane stream, means for providing a signal corresponding to a whole integer J, means for providing a signal corresponding to j which increases in unit steps until J is reached, means connected to the J signal means, to the j signal means, to the AR F signal means and to the sensing means for providing the trial isobutane flow rate R signals, in accordance with the R J, j and AR signals and the following equation:

and means connected to the j signal means for providing a pulse indicative of the end of current operation when j is substantially equal to J.

12. A system as described in claim 11 in which the selecting means includes means for storing the R sig-- nal, the R signal and the E signal, means connected to the storing means for comparing a stored E signal with the E signal from the profit change signal means and for clearing the storing means when the E signal from the profit change signal means is greater than the stored E signal and storing the R signal, E signal and the R signal and for not clearing the storing means when the E signal from the profit change signal means is not greater than the stored E signal so that the storing means contains the R and R signals for a maximum increase in profit, and means for providing the stored R and R signals as the control signals to the control means in response to an end pulse.

13. A method for controlling an alkylation .unit to achieve a desired operating condition, said alkylation unit includes a contactor wherein olefin and isobutane streams, entering the contactor, are contacted in the presence of acid and said contactor provides an acidhydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and acid, a portion of the separated acid is discharged while the remainder of the separated acid along with fresh acid entering the alkylation unit is recycled to the contactor which comprises controlling the isobutane flow rate and one acid flow rate of the fresh acid and discharge acid flow rates relative to the other acid fiow rate in accordance with control signals; sensing certain operating parameters of the alkylation unit; providing signals corresponding to the sensed parameters; providing a signal corresponding to trial isobutane flow rates R providing a signal corresponding to trial one acid flow rates R associated with the trial isobutane flow rates; determining an anticipated change in profits associated with the implementation of each trial isobutane flow rate and its corresponding one acid flow rate andproviding a corresponding signal, said profit change determining step includes providing signals corresponding to economic values D, D and D associated with the isobutane, the acid and the octane rating of the alkylate, respectively, providing a signal corresponding to a change AQ in octane rating, providing a signal corresponding to a change AA in acid consumption, providing a signal corresponding to a change AM in the isobutane, and providing the profit change E signal in accordance with the signals from the AC), AA, AM, D D, and D signals and signals from the sensing means corresponding to the flow rate R of the hydrocarbon product and the volume fraction V of alkylate in the hydrocarbon product and the following equation:

15. A method as described in claim 14 in which the step of providing the G signal includes providing a signal corresponding to a quantity S providing a signal corresponding to a quantity S providing a signal corresponding to a calculated fraction volume V of isobutane in the hydrocarbon product, providing the 0,, signal in accordance with the 5,, signal, a signal corrresponding to the sensed volume fraction V of isobutane in the hydrocarbon product and the following equation:

where b,', b and b are constants, and providing the G signal in accordance with the S(' and V signals and the following equation:

GC= blSC I721! lV 16. A method as described in claim 13 in which the step of providing the signal includes providing a signal corresponding to the existing acid consumption A providing a signal corresponding to a quantity F providing a signal corresponding to a quantity F providing a signal corresponding to a quantity E providing a signal corresponding to a quantity F and providing a signal corresponding to the change AA in acid consumption in accordance with the A F F F F signals and the following equation:

AA n( sr im/ sa m' l7. A method as described in claim 16 in which the step of providing the A signal includes providing a signal corresponding to the acid consumption A in accordance with the signals corresponding to the sensed discharge acid flow rate R the sensed hydrocarbon product flow rate R and the sensed volume fraction V of alkylate in the hydrocarbon product and the following equation:

where u is a constant.

18. A-method as described in claim 17 in which the step of providing the F signal means includes providing a signal corresponding to the ratio P,, of propylene to olefins, and providing the F signal in'accordance with the S and P signals and the following equation:

ss P n 1( a/ n) z( n/ n) 3( n/ 4( B/ l n)*l where d through d, are constants, and the steps of providing the F signal includes providing the signal corresponding to the quantity F. in accordance with the S signal and the P signal and the following equation:

19. A method as described in claim 18 in which the step of providing the 5,, signal includes providing a signal corresponding to the hydrocarbon content H on the recycle acid in accordance with signals from the sensing means corresponding to the sensed densities D D and D corresponding to the densities of the fresh acid, of the recycle acid and of the hydrocarbon product, respectively, and the following equation:

and providing the signal corresponding to the quantity S,, in accordance with the H signal and signals corresponding to the sensed flow rates R R and R of the olefins, the recycle acid and the hydrocarbon product, respectively, and to the sensed volume fraction V of the olefins in the olefin stream and the following equation:

where Cy is the volume of the contactor, providing a signal corresponding to a calculated flow rate R for the discharge acid; and the step of providing the 8 signal includes providing the S signal in accordance with the R R R and V signals and the following equation:

20. A method as described in claim 19 in which the step of providing the F signal includes providing the F signal in accordance with a signal corresponding to the sensed volume fraction V of isobutane in the hydrocarbon product and the following equation:

ilm 12H) where c is a constant; and the step of providing the F signal includes providing a signal corresponding to a calculated volume fraction V of isobutane in the hydrocarbon product, and providing the F in accordance with the V signal and the following equation:

F Veg).

21. A method as described in claim 20 in which the step of providing the signal includes providing the V signal in accordance with the AM, R R V signals and a signal corresponding to the sensed volume fraction V of isobutane in the isobutane stream and the following equation:

22. A method as described in claim 15 in which the step of providing the AM signal includes providing a signal corresponding to AM for each trial isobutane flow rate in accordance with the trial isobutane flow rate signal and a signal corresponding to the sensed flow rate R of the isobutane and the following equation:

AM RMFR 23. The method as described in claim 13 in which the step of providing the isobutane flow rate signal R includes providing a signal corresponding to a maxi- J, j and AR signals and the following equation: mum flow rate change AR for the isobutane stream,

providing a signal corresponding to a whole integer J, Rm providing a signal corresponding to j which increases in and providing a pulse indicative of the end of current unit steps until J is reached, providing the trial isobu- 5 operation when j is substantially equal to J.

tane flow rate R signal, in accordance with the R 

2. A system as described in claim 1 in which the Delta Q signal means includes G signal means for providing signals corresponding to quantities GC and GB, and means for providing the Delta Q signal in accordance with the GC, GB signals and the following equation: Delta Q GC-GB.
 3. A system as described in claim 2 in which the G signal means includes means for providing a signal corresponding to a quantity SB, means for providing a signal corresponding to a quantity SC, means for providing a signal corresponding to a calculated fraction volume VCC of isobutane in the hydrocarbon product, means connected to the SB signal means and to the sensing means for providing the GB signal in accordance with the SB signal, a signal from the sensing means corresponding to the volume fraction VCB of isobutane in the hydrocarbon product and the following equation: GB b1SB2+b21n(100VCB+b3) where b1, b2 and b3 are constants, and means connected to the SC signal means and to the VCC signal means for providing the GC signal in accordance with the SC and VCC signals and the following equation: GC b1SC2+b21n(100VCC+b3)
 4. A system as described in claim 1 in which the Delta A signal means includes means for providing a signal corresponding to the existing acid consumption AB, means for providing a signal corresponding to a quantity FSC, means for providing a signal corresponding to a quantity FSB, means for providing a signal corresponding to a quantity FMB, means for providing a signal corresponding to a quantity FMC, and means connected to the AB, FSC, FSB, FMB and FMC signal means for providing a signal corresponding to the change Delta A in acid consumption in accordance with the AB, FSC, FMC, FSB, FMB signals and the following equation: Delta A AB (ESC FMB/FSBFMC - 1).
 5. A system as described in claim 4 in which the AB signal means includes means connected to the sensing means for providing a signal corresponding to the acid consumption AB in accordance with the signals from the sensing means corresponding to the discharge acid flow rate RAB, the hydrocarbon product flow rate RCB and the volume fraction VCK of alkylate in the hydrocarbon product and the following equation: AB aRAB/RCBVCK where a is a constant.
 6. A system as described in claim 5 in which the FSB signal means includes means for providing a signal corresponding to the ratio PB or propylene to olefins, and means connected to the SB and PB signal means for providing the FSB signal in accordance with the SB and PB signals and the following equation: FSB exp(SB2( d1(PB/1+PB) +d2(PB/1-PB)2 +d3(PB/1-PB)3 +d4(PB/1-PB)4)) where d1 through d4 are constants, and the FSC signal means includes means connected to the SC signal means and to the PB signal means for providing the signal corresponding to the quantity FSC in accordance with the SC signal and the PB signal and the following equation: FSC exp(SC2(d1(PB/1-PB) +d2(PB/1-PB)2 +d3(PB/1-PB)3 +d4(PB/1-PB)4))
 7. A system as desscribed in claim 6 in which the SB signal includes means connected to the sensing means for providing a signal corresponding to the hydrocarbon content H on the recycle acid in accordance with signals from the sensing means corresponding to the sensed densities DF, DB and DH corresponding to the densities of the fresh acid, of the recycle acid and of the hydrocarbon product, respectively, and the following equation: H (DF-DB)/(DF-DH)100 and means connected to the H signal means and to the sensing means for providing the signal corresponding to the quantity SB in accordance with the H signal and signals from the sensing means corresponding to the flow rates RO, RR and RCB of the oelfins, the recycle acid and the hydrocarbon product, respectively, and to the volume fraction VI of olefins in the olefin stream and the following equation: SB 100ROVT(RR+RCB)/CVRR(100 -H) where CV is the volume of the contactor, means for providing a signal corresponding to a calculated flow rate RCC for the discharGe acid; and the SC signal means includes means connected to the H signal means, to the sensing means and to the RCC signal means for providing in accordance with the RO, RR, RCC and VT signals and the following equation: SC 100ROVT(RR+RCC)/CVRR(100-H)
 8. A system as described in claim 7 in which the FMB signal means includes means connected to the sensing means for providing the FMB signal in accordance with a signal from the sensing means corresponding to the volume fraction VCB of isobutane in the hydrocarbon product and the following equation: FMB (100VCB)c where c is a constant; and the FMC signal means includes means for providing a signal corresponding to a calculated volume fraction VCC of isobutane in the hydrocarbon product, and means connected to the VCC signal means for providing the FMC in accordance with the VCC signal and the following equation: FMC (100VCC)c.
 9. A system as described in claim 8 in which the VCC signal means includes means connected to the RCC signal means, to the Delta M signal means and to the sensing means for providing the VCC signal in accordance with the Delta M, RCC, RCB, VCB signals and a signal from the sensing means corresponding to the sensed volume fraction VM of isobutane in the isobutane stream and the following equation: VCC ( Delta M)(VM) +RCBVCB/RCC.
 10. A system as described in claim 1 in which the Delta M signal means includes means connected to the trial isobutane flow rate signal means and to the sensing means for providing a signal corresponding to Delta M for each trial isobutane flow rate in accordance with the trial isobutane flow rate signal and a signal from the sensing means corresponding to the sensend flow rate RMB of the isobutane and the following equation: Delta M RMj-RMB.
 11. The system as described in claim 1 in which the trial isobutane flow rate signal means is connected to the sensing means and includes means for providing a signal corresponding to a maximum flow rate change Delta RMax for the isobutane stream, means for providing a signal corresponding to a whole integer J, means for providing a signal corresponding to j which increases in unit steps until J is reached, means connected to the J signal means, to the j signal means, to the Delta RMax signal means and to the sensing means for providing the trial isobutane flow rate RMj signals, in accordance with the RMB, J, j and Delta RMax signals and the following equation: RMj RMB+((2j/J) - 1 )( Delta RMax), and means connected to the j signal means for providing a pulse indicative of the end of current operation when j is substantially equal to J.
 12. A system as described in claim 11 in which the selecting means includes means for storing the RMj signal, the RAC signal and the EC signal, means connected to the storing means for comparing a stored EC signal with the EC signal from the profit change signal means and for clearing the storing means when the EC signal from the profit change signal means is greater than the stored EC signal and storing the RMj signal, EC signal and the RAC signal and for not clearing the storing means when the EC signal from the profit change signal means is not greater than the stored EC signal so that the storing means contains the RMj and RACj signals for a maximum increase in profit, and means for providing the stored RMj and RACj signals as the control signals to the control means in response to an end pulse.
 13. A method for controlling an alkylation unit to achieve a desired operating condition, said alkylation unit includes a contactor wherein olefin and isobutane streams, entering the contactor, are contacted in the presence of acid and said contactor provides an acid-hydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and acid, a portion of the separated acid is discharged while the remainder of the separated acid along with fresh acid entering the alkylation unit is recycled to the contactor which comprises controlling the isobutane flow rate and one acid flow rate of the fresh acid and discharge acid flow rates relative to the other acid flow rate in accordance with control signals; sensing certain operating parameters of the alkylation unit; providing signals corresponding to the sensed parameters; providing a signal corresponding to trial isobutane flow rates RMj; providing a signal corresponding to trial one acid flow rates RACj associated with the trial isobutane flow rates; determining an anticipated change in profits associated with the implementation of each trial isobutane flow rate and its corresponding one acid flow rate and providing a corresponding signal, said profit change determining step includes providing signals corresponding to economic values DM, DA and DQ associated with the isobutane, the acid and the octane rating of the alkylate, respectively, providing a signal corresponding to a change Delta Q in octane rating, providing a signal corresponding to a change Delta A in acid consumption, providing a signal corresponding to a change Delta M in the isobutane, and providing the profit change EC signal in accordance with the signals from the Delta Q, Delta A, Delta M, DQ, DA and DM signals and signals from the sensing means corresponding to the flow rate RCB of the hydrocarbon product and the volume fraction VCK of alkylate in the hydrocarbon product and the following equation: EC ( Delta Q)(DQ)-( Delta A)(DA)-Delta MDM/RCBVCK; and selecting the trial isobutane flow rate and its corresponding one acid flow rate that provides for the greatest increase in profits in accordance with the profit signal and providing the selected signals as the control signals to the control means.
 14. A method as described in claim 13 in which the step of providing the Delta Q signal includes providing signals corresponding to quantities GC and GB, and providing the Delta Q signal in accordance with the GC, GB signals and the following equation: Delta Q GC-GB.
 15. A method as described in claim 14 in which the step of providing the G signal includes providing a signal corresponding to a quantity SB, providing a signal corresponding to a quantity SC, providing a signal corresponding to a calculated fraction volume VCC of isobutane in the hydrocarbon product, providing the GB signal in accordance with the SB signal, a signal corrresponding to the sensed volume fraction VCB of isobutane in the hydrocarbon product and the following equation: GB b1SB2+b21n(100VCB+b3) where b1, b2 and b3 are constants, and providing the GC signal in accordance with the SC and VCC signals and the following equation: GC b1SC2+b21n(100VCC+b3).
 16. A method as described in claim 13 in whicH the step of providing the signal includes providing a signal corresponding to the existing acid consumption AB, providing a signal corresponding to a quantity FSC, providing a signal corresponding to a quantity FSB, providing a signal corresponding to a quantity FMB, providing a signal corresponding to a quantity FMC, and providing a signal corresponding to the change Delta A in acid consumption in accordance with the AB, FSC, FMC, FSB, FMC signals and the following equation: Delta A AB(FSCFMB/FSBFMC -1).
 17. A method as described in claim 16 in which the step of providing the AB signal includes providing a signal corresponding to the acid consumption AB in accordance with the signals corresponding to the sensed discharge acid flow rate RAB, the sensed hydrocarbon product flow rate RCB and the sensed volume fraction VCK of alkylate in the hydrocarbon product and the following equation: AB aRAB/RCBVCK where a is a constant.
 18. A method as described in claim 17 in which the step of providing the FSB signal means includes providing a signal corresponding to the ratio PB of propylene to olefins, and providing the FSB signal in accordance with the SB and PB signals and the following equation: FSB exp SB2(d1(PB/1-PB) +d2(PB/1-PB)2 +d3(PB/1-PB)3 +d4(PB/1-PB)4) where d1 through d4 are constants, and the steps of providing the FSC signal includes providing the signal corresponding to the quantity FSC in accordance with the SC signal and the PB signal and the following equation: FSC exp SC2) d1(PB/1-PB) +d2(PB/1-PB)2 +d3(PB/1-PB)3 + d4(PB/1-PB)4).
 19. A method as described in claim 18 in which the step of providing the SB signal includes providing a signal corresponding to the hydrocarbon content H on the recycle acid in accordance with signals from the sensing means corresponding to the sensed densities DF, DB, and DH corresponding to the densities of the fresh acid, of the recycle acid and of the hydrocarbon product, respectively, and the following equation: H (DF-DB)/(DF-DH)100, and providing the signal corresponding to the quantity SB in accordance with the H signal and signals corresponding to the sensed flow rates RO, RR, and RCB of the olefins, the recycle acid and the hydrocarbon product, respectively, and to the sensed volume fraction VT of the olefins in the olefin stream and the following equation: SB 100ROVT(RR+RCB)/CVRR(100- H) where CV is the volume of the contactor, providing a signal corresponding to a calculated flow rate RCC for the discharge acid; and the step of providing the SC signal includes providing the SC signal in accordance with the RO, RR, RCC and VT signals and the following equation: SC 100ROVT(RR+RCC)/CVRR(100- H).
 20. A method as described in claim 19 in which the step of providing the FMB signal includes providing the FMB sigNal in accordance with a signal corresponding to the sensed volume fraction VCB of isobutane in the hydrocarbon product and the following equation: FMB (100VCB)c where c is a constant; and the step of providing the FMC signal includes providing a signal corresponding to a calculated volume fraction VCC of isobutane in the hydrocarbon product, and providing the FMC in accordance with the VCC signal and the following equation: FMC (100VCC)c.
 21. A method as described in claim 20 in which the step of providing the signal includes providing the VCC signal in accordance with the Delta M, RCC, RCB, VCB signals and a signal corresponding to the sensed volume fraction VM of isobutane in the isobutane stream and the following equation: VCC ( Delta M)(VM) +RCBVCB/RCC.
 22. A method as described in claim 15 in which the step of providing the Delta M signal includes providing a signal corresponding to Delta M for each trial isobutane flow rate in accordance with the trial isobutane flow rate signal and a signal corresponding to the sensed flow rate RMB of the isobutane and the following equation: Delta M RMj-RMB.
 23. The method as described in claim 13 in which the step of providing the isobutane flow rate signal RMj includes providing a signal corresponding to a maximum flow rate change Delta RMax for the isobutane stream, providing a signal corresponding to a whole integer J, providing a signal corresponding to j which increases in unit steps until J is reached, providing the trial isobutane flow rate RMj signal, in accordance with the RMB, J, j and Delta RMax signals and the following equation: RMj RMB + (2j/J -1) ( Delta RMax), and providing a pulse indicative of the end of current operation when j is substantially equal to J. 